|A publication of the National Electronics Manufacturing Center of Excellence||
Michael D. Frederickson
The U.S. Navy's next generation surface ships will utilize various power systems, such as Integrated Fight Through Power (IFTP), Electric Drive (ED), and Reconfigurable Zonal Systems. The present on-board power stations, however, are not adequate to support electric weapons or high power radar. A fundamental change in how electric power is converted, distributed, and managed will be required in order to fully utilize the electric power available aboard these new platforms. The concept of an "electric warship" will depend on the ability to rapidly shift power to major loads to support tactical needs. To ensure this capability can be met in a timely and affordable manner, there are technology issues that must be addressed at both the material and component levels.
One issue that must be addressed involves weight and size. Weight and center of gravity are critical concerns of the next generation power distribution system. The current methods for generating pulsed power require the use of heavy, bulky systems placed in areas that are not optimal for carrier design. One method to reduce the weight and size of the current iron and copper transformers used in legacy power distribution systems is to increase the switching frequency from 60 Hz to 15 - 20 kHz. The Solid State Power Station (SSPS), for example, uses this design principle. However, silicon exhibits high switching losses at these frequencies, so high power devices (e.g. Insulated Gate Bipolar Transistors - IGBTs) cannot operate at 10 - 20 kHz.
The new Wide Band Gap semiconductor materials, principally Silicon Carbide (SiC), offer superior materials properties to meet the higher power performance challenges. Continuous power switches, power diodes, and pulsed power switches fabricated from SiC offer reductions in on-state resistance and switching loss over conventional silicon power devices planned for future ship construction. For a given power rating, these components can operate at a higher duty cycle, leading to a reduction in the size of inductors and transformers in power circuits. SiC power electronics also extend solid state technology by offering higher breakdown voltage levels than current silicon technology, to address voltage levels presently managed by electromechanical switch technology.
Wide Band Gap semiconductors also represent a possible paradigm shift in semiconductor power density. As shown in Table 5-1, SiC devices can operate at higher temperatures and thus require less cooling. The higher blocking voltages, as compared to silicon devices, allow for the design of smaller and simpler high voltage components. Improved thermal management of semiconductors and passive components through upgraded packaging would allow more current to be handled by a given device and lead to improved power density designs.
Power conversion equipment developed using SiC technology is projected to significantly reduce the workload and maintenance requirements for current and future carriers and is considered a critical step in achieving CVN 21 compliance with key performance parameters for weight reduction and ship stability. As indicated in Figure 5-1, use of SiC power conversion on Navy ships is expected to reduce the current conversion equipment size by approximately 60% and achieve weight savings approaching 2.68 tons for each converter implemented with the new 2.7 MVA transformer technology.
The current packages that are available for SiC severely limit the device applications. To realize the benefits of SiC device technology, designers and engineers need packaged components. However, nearly all component packaging materials are designed for use with silicon devices. Therefore, these packaging solutions were never intended for use above about 150ºC. Such packaging would negate a major advantage of SiC devices, namely their high operating temperature capability. Operation of SiC Shottkey diodes has been demonstrated above 350ºC. High temperature packaging options include both polymer-based and ceramic-based solutions. Currently, the vast majority of high power packages are manufactured using polymers. It is expected that compatible polymer material sets can be found to allow operation in the 200ºC - 250ºC range. For operation above 300ºC, ceramic packages are much more likely candidates. Above 300ºC diffusion, oxidation and corrosion are all much more active processes than at 150ºC, so new metallurgical interconnect solutions will be required for long-term reliability.
ACI is starting a ManTech project aimed at developing high power packages for use in the SSPS and other DoD applications. To be compatible with the commercial packaging infrastructure, these packages will be based on high temperature polymers. Operating temperatures are planned to be above 200ºC. Besides providing the I/Os between the system and the microelectronic devices, the device packaging will provide:
A high power component can consist of one or more die that are mounted on a substrate. High power components often use high thermal conductivity die attach materials to mount the die on the substrates. High temperature solders such as gold-tin or gold-germanium can be used. Table 5-2 shows the metrics that will be used to evaluate technology developed to support Solid State Power Substation for the "all-electric" warship. For best reliability, the die are attached to high thermal conductivity substrates that have a low Coefficient of Thermal Expansion (CTE). Potential substrate materials include: aluminum nitride, beryillia, alumina, copper-tungsten alloys, and aluminum silicon carbide metal matrix composites. The bottom-side of the substrate will later be attached to a high capacity heatsink or thermal management system to extract the heat generated by the high power component.
All die intended for high voltage components must have all sharp corners and edges rounded to eliminate electric field concentration effects. This is often performed using automated grinding equipment. High voltage die also require surface passivation to eliminate surface leakage currents that often result from surface damage caused by wafer cuting and polishing operations. Surface passivation is often accomplished by the application of a very uniform coating of polyimide directly on the bare device. Moisture resistance can also be achieved with a uniform, pinhole-free coating deposited on the device. Mechanical and corrosion protection are achieved through encapsulation with thicker layers of polymers. For corrosion protection, it is important that the encapsulation does not have any cracks or interconnected porosity that will allow ions to be transported to the device. Besides these layers, a packaged device might have other polymer structures that become part of the package. A power device package may be composed of a substrate, one or more die attached to a substrate with a die attach material, wirebonds, I/O leads or screws, and three or more different polymers. These different materials encompass a broad spectrum of properties. Component reliability is critically dependent on the compatibility of all materials that comprise the package.
When designing a high power package that will have large area die, a trade-off is always necessary to balance the CTE mismatch between the various materials that comprise the package. High power packages are designed to carry current pulses in the range of 100A - 100,000 A. This forces the package to have relatively large conductors made from high conductivity metals such as copper. All high conductivity metals have high CTEs (e.g., the CTE of copper is 16 ppm/ºC). However, microelectronic devices have a low CTE (e.g., the CTE of SiC is 4 ppm/ºC), which creates significant packaging issues for devices expected to be routinely cycled between room temperature and operating temperatures exceeding 200ºC. For packages to be reliable, they must remain crack-free after undergoing many thermal cycles. As devices are designed for higher and higher operating temperatures, the thermal cycling stresses become larger and larger. Compatibility must be ensured by testing high power components to failure and understanding the failure modes. Since the CTE mismatches in a package are quite substantial, the materials must be compliant. In addition, all interfaces within the structure must exhibit excellent wetting properties so high adhesion strength is developed. In high voltage components, small cracks (especially near metallic conductors) are a source of partial discharge events, which will eventually lead to dielectric breakdown failure.
In conjunction with a commercial supplier of packages, ACI expects to begin a Navy ManTech project to develop high temperature packaging for high power SiC devices. The program will ascertain a compatible materials set and will develop a reliability model for the package.
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